The use of intermediate toner transfer members in electrostatography has been suggested for several reasons, including simplified receiver sheet handling, single pass duplexing, reduced wear of photoconductors, and superposition of multiple images to form multicolor images. Typically, a toner image is created on a photoconductive member electrostatographically and is transferred by conventional, electric field assisted processes to an intermediate roller or web. For example, a negatively charged toner image is transferred from a photoconductor having a grounded backing electrode to an intermediate web or roller biased to a strong positive polarity. The toner image is then transferred from the intermediate member to a receiver sheet under the influence of a second electric field that can be created without changing the field on the intermediate member by placing a roller or a corona behind the receiver sheet that is biased still more strongly in a positive direction.
Of the reasons mentioned above for the use of intermediate toner transfer members, probably the most important use relates to the formation of multicolor images. For this application, two, three, or four separate images of different color can be transferred in registration to the intermediate transfer member to form a multicolor image that can then be transferred in one step to a receiver sheet. This method has several advantages over the approach in which the receiver sheet is secured to the periphery of a roller and rotated repeatedly into transfer relation with the photoconductor to receive the separate color images directly. Probably the most important advantage is that the receiver sheet itself does not have to be attached to a roller, which is a source of image misregistration and apparatus complexity. Other advantages associated with wear and tear on the photoconductive element and a direct receiver sheet path are also important.
As color electrostatography, especially electrographic color printing, continues to improve, increasingly higher image resolution will be required. In order to obtain high resolution, toners of fine particle size are necessary. Toners with particle size less than 20.mu., and especially those less than 10.mu. in size, give substantially improved resolution in color imaging with high quality equipment.
Unfortunately, fine particle toners are more difficult to transfer electrostatically than more traditional coarse toners. This is a problem in conventional electrostatography utilizing a single transfer of fine toner particles. It is a substantially more difficult problem using an intermediate transfer member in color electrostatography, which entails two transfers each of a plurality of different color images.
An intermediate toner transfer member typically includes a substrate on which is formed a relatively thick, resilient blanket and a relatively thin, hard outer layer on the blanket. The blanket, which may be integral with the substrate, is formed from a compliant polymeric material, frequently a polyurethane, that facilitates contact of the toner particles with the member during the transfer process. The blanket may be electrically modified to enhance the electrostatic attraction of the toner particles. Because compliant materials such as polyurethanes do not release toner very well, a relatively thin, hard surface layer is applied over the blanket layer.
Several properties of the intermediate transfer member surface are especially important. First, the surface energy must be sufficiently low to facilitate release of the fine toner particles, whose diameter may be on the order of 3-4.mu.. In addition, the intermediate member surface must have good wear properties against the highly abrasive conditions of the transfer process. During transfer, pressure is exerted on the particles of toner and, optionally, carrier at the first nip formed by the photoconductor and intermediate transfer member. Even higher pressure is typically exerted at the second nip, where the receiver, most often paper, is brought into contact with the toner on the intermediate transfer member surface. Residual toner is removed at a cleaning station that may include a blade, a fur brush, or a magnetic brush.
The material comprising the transfer member surface must also have sufficient flexibility to prevent cracking during the transfer process just described. The hardness of the substrate and blanket upon which the overcoat is applied can vary over a considerable range, so it is necessary to adjust the flexibility of the overcoat appropriately. Finally, the intermediate transfer member surface layer must be sufficiently thin to prevent its acting as an insulator against the development of the field necessary for electrostatic attraction of the toner particles. It must also not counteract the compliant properties of the material constituting the blanket of the transfer member.
In summary, it is very important to control the surface energy, wear, and flexibility properties of the transfer member surface layer. These properties can be evaluated by, respectively, contact angle measurements, abrasion test measurements, and storage modulus determinations.
U.S. Pat. No. 5,084,735, the disclosure of which is incorporated herein by reference, describes an intermediate transfer member whose thin, relatively hard outer skin has a Young's modulus greater than 5.times.10.sup.7 newtons/m.sup.2.
U.S. Pat. No. 5,337,129 discloses an intermediate toner transfer component having a substrate coated with a composition comprising integrated, interpenetrating networks of haloelastomer, preferably fluoroelastomer, and silicon oxide and, optionally, polyorganosiloxane.
U.S. Pat. No. 5,480,938 describes a low surface energy material comprising an elastomer composition that is a substantially uniform integral interpenetrating network of a hybrid graft composition of a fluoroelastomer and a polyorganosiloxane.
An intermediate transfer member having a blanket member overcoated with a thin layer of a thermoplastic, a sol-gel, or, preferably, a ceramer is described in co-pending, commonly assigned U.S. application Ser. No. 08/653,518, filed May 24, 1996, by Rimai et al., ELECTROSTATOGRAPHIC APPARATUS AND METHOD FOR IMPROVED TRANSFER OF SMALL PARTICLES, the disclosure of which is incorporated herein by reference, now U.S. Pat. No. 5,728,496.
There is a continuing need for an electophotographic intermediate toner transfer member whose surface exhibits superior properties of wear, surface energy, and flexibility. The present invention meets this need.